1. Field of the Invention
The present invention relates to a shift control system of an automatic transmission for vehicles.
2. Description of the Prior Art
An automatic transmission for vehicles achieves a plurality of gear positions providing respective predetermined gear ratios by engaging and disengaging engagement devices such as clutches and brakes combined with planetary gear mechanisms and actuated by hydraulic pressure.
To engage the engagement device, hydraulic pressure applied to the engagement device must be controlled in such a manner as to quickly complete a shifting operation while preventing a shift shock.
Accordingly, Japanese Laid-Open Patent Publication No. 2000-81125, for example, discloses a shift control system that proceeds a shifting operation while maintaining a command value of hydraulic pressure supplied to an engagement device at a constant shelf pressure at the initial stage of an inertia phase wherein the gear ratio represented by the ratio of input revolutions to output revolutions of a transmission changes from a before-shifting gear ratio toward an after-shifting gear ratio, and provides feedback control such that an actual gear ratio is equal to a target gear ratio at the later stage of the inertial phase.
Incidentally, an engagement device has driving and driven friction elements that are engaged or disengaged by hydraulic pressure. Even if the supply of hydraulic pressure to the engagement device starts in response to a shifting command, the engagement of the friction elements cannot be started immediately. The friction elements cannot actually start engaging with each other until a hydraulic pressure chamber of the engagement device is filled with working oil. It is therefore necessary to pass through a standby phase before the gear ratio starts changing.
Usually, in the standby phase, a hydraulic pressure command value is increased once to a high level in response to a shifting command at a time t0, and is then maintained at a low level for a predetermined period of time after the time t1 as shown in FIG. 8. The hydraulic pressure command value is then increased at a predetermined inclination from the time t2 to a time t3 when the gear ratio starts changing. The above-mentioned inertia phase lies after from the time t3 downward.
The time t2 is supposed to be a point in time when the hydraulic pressure chamber is filled with hydraulic fluid, and a broken line indicates an actual value of hydraulic pressure.
Upon completion of filling in the hydraulic pressure chamber, a surge S occurs in the actual value of the hydraulic pressure applied to the hydraulic pressure chamber with respect to hydraulic pressure command value in the standby phase from the time t0 to t3.
If the hydraulic pressure command value is too large in the standby phase, the actual filling is completed before the time t2 as shown in FIG. 9(a), and a shock occurs due to a great surge S resulting from a great difference between the hydraulic pressure command value and the hydraulic pressure actual value.
On the other hand, if the hydraulic pressure command value is too small in the standby phase, the filling is not actually completed when the hydraulic pressure should be increased toward an inertia phase starting point according to the hydraulic pressure command value as shown in FIG. 9(b), and this increases a period of time required for shifting.
A conventional shift control system, however, provides feedback control only in the inertia phase, and thus, a shock may occur or a shift time may be shifted in a shifting operation due to the above described phenomenon in the standby phase. This deteriorates the commercial value of the automatic transmission.
It is therefore an object of the present invention to provide a shift control system of an automatic transmission that is capable of properly controlling hydraulic pressure in a standby phase of an engagement device and quickly carrying out a shifting operation while preventing the occurrence of a shock.
To accomplish the above object, the present invention provides a hydraulic control system of an automatic transmission which outputs a hydraulic pressure command value increased to a high level, maintained at a low level for a predetermined period of time and then increased at a predetermined inclination until a gear ratio starts changing and engages an unengaged engagement device in response to a shifting command for shifting from a first gear ratio to a second gear ratio according to an operating state, the hydraulic control system comprising: standby phase target time setting means for setting a target time from an output of the shifting command until the gear ratio starts changing due to the supply of hydraulic pressure to the engagement device; standby phase real time measuring means for measuring a real time until the gear ratio starts changing due to the supply of hydraulic pressure to the engagement device after the output of the shifting command; correction value calculating means for calculating a correction hydraulic pressure value based upon a difference between the target time and the real time; and hydraulic pressure command value correcting means for correcting the hydraulic pressure command value by the correction hydraulic pressure value in a next shifting operation.
According to the present invention, the correction hydraulic pressure value is calculated based on a difference between the target time set by the standby phase target time setting means and the real time of the standby phase, and the command value of the hydraulic pressure applied to the engagement device is corrected by the correction hydraulic pressure in the next shifting operation. This compensates a change in characteristics of a hydraulic system for the engagement device, and prevents the occurrence of a surge shock and a shift delay.
In one preferred form of the present invention, the standby phase target time setting means sets a target time in each small region defined by an engine load and a vehicle speed, each small region where the vehicle speed increases while the engine load is substantially constant.
Since the standby phase target time setting means sets the target time when the operating state lies in a small region where the engine load is changing at a substantially constant rate and the vehicle speed is increasing, i.e. when the operating state is stable, there is no affection by a great change in revolutionary speed and a change in torque caused by a rapid change in operating state. This enables learning correction of a standby phase time by precisely detecting a change in characteristics of the hydraulic system and accurately correcting the hydraulic pressure command value.
In another preferred form of the present invention, the higher the vehicle speed is, the longer is the set target time.
By this, it is possible to correspond to the characteristics of the filling of hydraulic fluid pressure in the engagement device in a normal shifting operation. This surely prevents the occurrence of a surge shock and a shift delay.
In yet another preferred form of the present invention, the hydraulic control system further comprises: oil temperature detecting means for detecting an oil temperature; wherein the hydraulic pressure command value correcting means corrects a predetermined basic hydraulic pressure command value by the correction hydraulic pressure value; and the standby phase target time setting means sets the target time according to the oil temperature detected by the oil temperature detecting means.
With this feature, it is possible to control the shifting operation according to the effect of the viscosity, etc. resulting from the temperature of hydraulic fluid.
In yet another preferred form of the present invention, the lower the oil temperature is, the larger is the set basic hydraulic pressure command value. In yet another preferred form of the present invention, the lower the oil temperature is, the longer is the set target time. In yet another preferred form of the present invention, the lower the oil temperature is, the larger is the set correction hydraulic pressure value.
With the basic hydraulic pressure command value, the target time, or the correction hydraulic pressure value being determined to be larger, longer, or larger for a lower oil temperature, the delicate and accurate shifting operation can be achieved.
In yet another preferred form of the present invention, the next shifting operation includes a shifting operation outside the small region where the target time used for calculating the present correction hydraulic pressure value is set.
Since the correction hydraulic value is determined according to a change in characteristics of the respective components in the hydraulic system concerned with the engagement device, the hydraulic pressure command value can be corrected by directly using the correction hydraulic value for shifting in an operating state outside the small region that is used for calculating the correction hydraulic pressure value.